Hammer versus press forming, kinetic energy vs pressure

[BenStorch]

Hello,

I am trying to work out the difference in force exerted by either a press or a power hammer, for a sheet metal forming application.
I worked out that the kinetic energy of a hammer is calculated as E = ½mv².
Example of power hammer: 0.5 * 30 kg * (15m/sec)sq = 3.375 KJ

How does this relate to the force exerted by a 20 ton press, where the momentum/velocity doesn't really matter that much, or does it, if it is only 0.01 m/s ?
20 ton Press : 0.5 * 20000 kg * (0.01m/sec)sq = 1 KJ

That doesn't make sense, does it ?

There must be another force comparison between pressure and kinetic energy.

What about this relation :
1 tonne-force = 9806.65 N = 9806.65 joule/meter?
So the 20 ton press would exert a force of 196.13 KJ/meter ?
Still doesn't seem enough in comparison with the hammer blow.

Can anyone help me with this ?

Thanks, Benjamin

 

[desertfox]

Hi Benstorch

Pressure and energy are two diffent things units for energy are Joules =(kgm^2/s^2) or (Nm), pressure on the other hand is measured in N/m^2.
Now a constant force acting over a travelled distance ie:- Force * Distance = Energy, so if you consider dropping a mass of 10kg through a vertical height of say 5m we can say:-


mass*g*h= 10*9.81*5= 490.5 Nm or joules

Now assuming no losses you can say that the energy used in dropping the stone from 5m is converted into kinetic energy
therefore:-

mass*g*h=0.5*mass*V^2

[490.5/(0.5*10)]^0.5 = V = 9.9 m/s

Hope this helps

regards

desertfox

 

[metengr]

BenStorch;
Simply stated, you cannot directly compare the force used in metal forming operation of presses. For hammer and screw presses, their main capacity is work (energy). A hammer's work capacity is determined from the ram mass and drop height. For a screw press, the work capacity is stored in the rotating masses.

For a hydraulic press, the deformation force is directly controlled through adjusting pressure.

Source; Metal Forming Practise
by Heinz Tschaetsch

 

[Pressed]

For some really slow hydraulic presses you can see the ram stop once it touches the workpiece, then it builds up pressure, overcomes the workpiece then continues traveling. Sometimes it is hard to detect the stop in motion or the speed is negligible. These small and simple hydraulic presses are pure pressure.

===================================
Forging

http://mech.e.tripod.com

-------------------------------------------------
Ddraig Breuddwydia....

http://www.tailofthedragon.com

 

[CoryPad]

The machines transfer their energy to the material. The material has then been worked. Work equals energy. Work is a function of stress and strain. You can calculate stress and strain on a workpiece, then calculate work, then compare to energy from a machine.

 

[BenStorch]

Thank you all for your comments,
it is interesting to see how many perspectives there are on the subject.
So it seems there is no straightforward equation to relate the two types of forces/loads applied in the two forming processes.
I guess the difference in time, between the impulsive hammer force and the steady pressure of a press, is what makes the comparison difficult.
As CoryPad suggested, maybe it's worth setting up an experiment, where identical pieces of metal are subjected to either process and the resulting deformation is compared.
As to the application - I create negative, saddle-shaped curvature in sheet metal for sculpture, and after some experimentation am wondering which direction - to develop my tooling in - whether press or power hammer, or a heavy duty rolling press, like a pressurised, motorised English wheel.

Many thanks again - Benjamin

 

[Pressed]

Theoretically the two presses can cause the same force, which can be compared, you just can't use the same equation to derive them. The forces can be measured and compared, but the quality and accuracy of the measuring will add variables. Depending on certain factors, the outcome can be different from dynamic load vs static load, especially in sheet metal forming.

You can ballpark the press requirements, but if the press or metal forming parameters are operated at the edge of capability the best way to determine formability is at the press. What is the ballpark? Depends on the product and the presses, things you learn about your plant over-time.

You could compare the (force)*(time) area under the press curve, but that numerical data isn't really captured by any press; which leads back to variables of quality and accuracy in measuring. This would require the synchronized numerical measurement of the ram force, and duration of forming; throughout the entire forming operation to draw the area and make a comparison or estimation to the next press.

After that, you might have to add a formation timer to the hydraulic press to compare to the estimate and have an efficient stroke. These ideas require instrumentation and/or control which most companies don't have on their presses.


===================================
Forging

http://mech.e.tripod.com

-------------------------------------------------
Ddraig Breuddwydia....

http://www.tailofthedragon.com

 

[Robertmet]

The difference between press and hammer is obvious when forging large , 12"x12", tool steel pieces. The press works the center , the hammer doesn't !

 

[BenStorch]

Many thanks Pressed for your detailed explanation of all the factors involved in the comparison, I think I will stick to a hands-on comparison, considering the number of variables - but I like the sound of the 'force*time area under the pressure curve', I will keep that in mind.

And thanks Robertmet, can you tell me what kind of press you are using for forging the tool steel?
That sounds promising.

Cheers, Benjamin

 

[Robertmet]

This goes back many years so I don't remember the details . We were supplying the tool steel.There was one forger who only had hammers yet insisted on taking on large piece projects.He always had problems .Other forgers who had presses never had the same problems.Of course tool steels of large x-section are never homogeneous in the center and any working of the center is a good idea !!